In calibration of thermometers, the freezing points or melting points of metals are used as defining fixed-points of temperature in the range above room temperature, and a fixed-point cell is used as means for providing the fixed-points. Generally, a graphite crucible containing a cast pure metal as a fixed-point material is used as such a fixed-point cell. A thermometer is calibrated by measuring the temperature inside the fixed-point cell with the thermometer. Specifically, the fixed-point cell is placed inside a variable temperature furnace, and the changes in temperature of the cell when its ambient temperature is increased and decreased are observed. In a state in which the liquid and solid phases of the fixed-point material coexist, the temperature change does not occur due to the latent heat of fusion. The thermometer is calibrated by utilizing this phenomenon (see Non-Patent Document 1).
For example, Patent Document 1 discloses a fixed-point cell including: a crucible composed of carbon; and a fixed-point material enclosed in the crucible and having a eutectic structure of carbon and a metal. Moreover, Patent Document 2 discloses a fixed-point cell including: a crucible composed of carbon; and a fixed-point material enclosed in the crucible and having a eutectic structure of carbon and a carbide or a eutectic structure of carbon and a solid solution of carbon.
However, with the fixed-point cell of any of the above Non-Patent Document 1 and Patent Documents 1 and 2, when the ambient temperature of the crucible placed inside a variable temperature electric furnace is increased and decreased for the purpose of thermometer calibration, the fixed-point material is expanded or contracted, and stress is applied to the crucible. This may cause cracks in the crucible.
Meanwhile, in Non-Patent Document 2, the present inventor has presented a fixed-point cell including layers of a plurality of pyrolytic graphite sheets which is inserted into the gap between a crucible and a fixed-point material composed of a porous material.
However, in the course of ongoing research on the fixed-point cell, the inventor has found the following. The fixed-point cell has a graphite support inserted therein and formed by laying the pyrolytic graphite sheets. The pyrolytic graphite sheets are difficult to purify and contain about 0.1% of phosphorus as an impurity. Therefore, when the fixed-point cell is placed in a variable temperature electric furnace and the ambient temperature of the cell is increased and decreased for the purpose of thermometer calibration, phosphorus diffuses into the fixed-point material in contact with the pyrolytic graphite sheets. Hence, the fixed-point material is contaminated with phosphorus, so that the reproducibility of thermometer calibration decreases significantly.
Patent Document 1: Japanese Patent No. 2987459.
Patent Document 2: Japanese Patent No. 3404531.
Non-Patent Document 1: Chapter 7 of “Proper Use of Thermometers (revised edition),” edited by Japan Electric Instrument Manufacturers' Association, published by Japan Industrial Publishing Co., Ltd., (1997).
Non-Patent Document 2: Y. Yamada and P. Bloembergen, “High-Temperature Metal-Carbon Eutectic Fixed-Point Cells with Improved Robustness”, SICE Annual Conference in Sapporo, Aug. 4-6, 2004, pp. 1027-1030.